472 Proceedings of the Royal Irish Academy. 



■under conditions favourable for the development of metamorphic 

 action, must, more or less similarly to sedimentary rocks, be altered, 

 according to their susceptibilities into schist, gneiss, or granitic-gneiss. 

 Examples of granite changed into gneiss are hereafter given ; while 

 the change from gabbro and other whinstone into schist and gneiss 

 can be studied in most areas of metamorphic rocks. 



A good field for examining the relations between metamorphosed 

 sedimentary and eruptive rocks is found in Yar Connaught on western 

 Galway. Here, to the south of the Clifden and Oughterard valley, 

 there is to the westward a rock-tract, in part sub-metamorphic and in 

 part schist, made up of altered sedimentary rocks and their associated 

 eruptive rocks. This association of rocks, as followed eastward, 

 becomes more and more gneissoid, till eventually all lose their indivi- 

 duality, and become merged into the granitic-gneiss and the meta- 

 morphic-granite of the barony of Moycullen. Also, in Yar Connaught 

 and the neighbouring county of Hayo, are exhibited good examples of 

 intrudes of felspathic rocks changed in part into schist and in part 

 into granitoid rocks ; while elsewhere are masses of basic eruj)tive 

 rocks which, when altered, change into different kinds, ranging from 

 hornblendite (hornblende-schist) to syenite and hornblendic-granite. 



It is not necessary that each successive invasion of regional meta- 

 morphism should act solely on that previously affected. On the 

 contrary, in some cases the newer action affects larger areas; but 

 more usually it appears to be confined to smaller limits. The latter, 

 however, may not be correct; because if, adjoining a tract of more 

 altered rocks, there are less altered ones, it is apparent that the first 

 invasion may have affected the small area, and the second the larger 

 one, or vice versa. In some cases, however, the weight of evidence 

 would suggest that the smaller areas are due to the effects of the later 

 invasion, as in them there are fewer faults and other breaks, many 

 having been sealed up during the newer invasion. 



Rocks successively altered, whether by regional or contact meta- 

 morphism, separately or combined, or even by methylotic action, have, 

 in general more or less marked, often hard boundaries ; in the latter 

 cases, as if the action had been stopped or limited by faults on some 

 such lines. Such hard, abrupt boundaries have led different observers 

 to consider the more altered rocks to be older than those with which 

 they are associated ; while, in most instances, consideration and 

 examination would prove the opposite to be correct, as the action 

 which has given the more altered rocTcs their present strongly -marked 

 characters took place at a time long subsequent to the accumulation of the 

 adjoining rocks, lohich at the present time are less altered. 



The sections across the tracts of the more altered Irish rocks are 

 so very similar, that the following may be given : — First, schist, 

 wi th subordinate limestone ; second, alternations of schist and gneiss ; 

 third, gneiss, with subordinate schists ; fourth, granitoid-gneiss (that 

 is, a granitic rock in tvhich the gneissoid characters are well marked), 

 with a few subordinate beds of schist and perhaps limestone ; fifth, 



